1. Field of the Invention
The present invention relates to a relief valve for use in a hydraulic overload protector of a mechanical press, and more specifically to an equipment for detecting an operation condition of the relief valve.
2. Disclosure of Prior Art
One of such conventionally known relief valve operation detectors is disclosed in Japanese Patent Publication No. 42-14430.
In this prior art, an operation cam is fixedly secured to a relief member to be actuated for relief in its axial direction, and the operation cam is kept in direct contact with a roller of a limit switch of the roller and lever type. When the relief member performs the relief operation, the cam is moved in the axial direction to swing a swinging lever of the limit switch at a predetermined angle, so that the relief operation condition can be detected.
Now, recently, since both the remarkable increasing of an operation speed and downsizing of a processing machine such as a mechanical press progress simultaneously, an improvement of operation accuracy and a downsizing are strongly required also of a relief valve for use in the processing machine.
In order to improve the operation accuracy of the relief valve, it is necessary to decrease a valve opening lift of the relief member. That is, since a time needed for the relief member to move from its closed position to its fully opened position becomes shorter due to the decreasing of the valve opening lift, a differential between a pressure at the time of the relief starting and a pressure at the time of the entire relieving becomes smaller correspondingly thereto. Therefore, it is possible to set a so-called cracking pressure accurately.
Further, also in order to manufacture the relief valve compactly, it is necessary to decrease the valve opening lift of the relief member similarly as mentioned above. That is, since a movement distance of the relief member becomes shorter, a housing etc. can be manufactured smaller in size correspondingly thereto.
Incidentally, in the case of the relief valve of the spring valve closing type, since also a spring lift becomes smaller by the decreasing of the valve opening lift, a service life of the spring becomes elongated.
As mentioned above, in order to improve the operation accuracy of the relief valve, it is necessary to make the valve opening lift of the relief valve as small as possible. But, when the valve opening lift is made small in that way, the following problems appear in such a construction that the the swinging lever of the limit switch is operated by the operation cam fixedly secured to the relief member like in the above-mentioned prior art.
That is, as the valve opening lift of the relief member is made smaller, also the movement distance of the operation cam becomes shorter. Therefore, it becomes impossible to swing the swinging lever at a desired angle. As a result, it becomes impossible to detect the condition of relief operation of the relief valve.
In order to solve the above-mentioned problems of the prior art, the inventor of the present invention proposed a detector illustrated in FIG. 7 preparatory to the present invention. This earlier invention has the following construction.
relief member 129 is inserted into a housing 120 so as to be movable in the right and left directions, a support member 150 is threadably secured to a right end portion of the relief member 129, a detected portion 165 like a thin ring is formed in the support member 150, and a detecting portion 168 of a proximity switch 114 is made to face an outer peripheral surface of the detected portion 165. The illustrated condition shows a normal condition in which the relief member 129 doesn't perform the relief operation. At the time of the relief operation, the relief member 129 and the detected portion 165 firstly move to the right relief position by a distance (e) in maximum and then move to the left relief completion position by a distance (f) via the illustrated normal position. Incidentally, in this earlier invention, since the valve opening lift of the relief member 129 is set to as small a value as possible, right and left movement distances of the detected portion 165 take very small values such as (e)=4.5 mm and (f)=2.5 mm.
When such very small rightward and leftward movements of the detected portion 165 are detected by the proximity switch 114, it is necessary to reduce a width dimension (w) of the detected portion 165 [herein, (w)=1.2 mm] and to maintain a clearance (h) between the detected portion 165 and the detecting portion 168 in a predetermined value [herein, (h)=0.8 mm].
But, in order to control the value of the clearance (h) accurately, it is necessary to finish the housing 120, a supporting bracket 153 of the proximity switch 114, the support member 150, the detected portion 165 and so on especially accurately and to assemble them elaborately in consideration of the existence of fitting clearances between those component parts. As a result, a manufacturing cost of the relief valve increases enormously.